1. Field of the Invention
The present invention relates to field of display technology, more particularly, relates to a method for adjusting the local backlight brightness of direct backlight in a display device.
2. Description of the Related Art
Recently, the liquid crystal display screen (LCD) is widely used from hand-held player, cell phone with small screen to LCD TV and computer display with large screen, and thus plays more and more important role in daily life of human being. Along with that, the energy-consumption thereof increasingly arouses concern. Since the LCD itself cannot emit light, it needs a powerful light source to provide backlight. However, this kind of light sources, such as a cold cathode fluorescent lighting (CCFL) or a light-emitting diode (LED) widely used in a LCD TV, consume large electricity energy. For example, when a typical 3.5 inch hand-held player plays a video, the total power consumption is about 500 mW, in which the power consumption of LCD screen is about 300 Mw. That's to say, the power consumption of LCD screen is about 60% or more of that of the whole player. Considering a whole LCD sub-system, including a control circuit and a frame buffer, the percentage of power consumption of LCD screen in the total power consumption of the system is much higher. Hence, it is quite meaningful to reduce the power consumption of the LCD to save energy and benefit the green society. Since the backlight source is a predominant energy consumer of LCT screen, people has endeavored to reduce the power consumption of backlight source.
In prior art, the method for reducing the power consumption of the backlight source comprises improving a drive-circuit of the backlight source, improving a luminous efficiency of LED, developing a new kind of LED, and adjusting the backlight according to the brightness of environment, etc. However, the local backlight adjusting method is a method easy to implement and having a significant effect. Especially, the local backlight adjusting method for direct backlight has effects of significantly reducing power-consumption of LCD screen, improving the contrast value and gray level value of a display image and reducing ghost, etc.
Many local backlight adjusting methods have been proposed, such as a local backlight adjusting and compensating method for direct backlight. The flow chart thereof is shown as FIG. 3. According to the method, the whole backlight region facing the display panel is divided into a plurality of backlight sub-regions (generally, the backlight region has already been divided into a number of sub-regions when designing the direct backlight source). The method comprises analyzing the gray level of a frame of input image signal (input image), and obtaining the average gray level or weighted gray level of the pixels corresponding to each backlight sub-region; since the backlight intensity of each backlight sub-region is independent from each other, dynamic adjusting may be performed with respect to backlight in the respective backlight sub-regions according to different gray levels of image signals and the gray level of the image to be displayed; the object of backlight adjusting is to adjust the brightness of each backlight sub-region. While the backlight brightness is adjusted, the backlight adjusting brightness information or gray level information of each backlight sub-region needs to be output to a backlight brightness simulating unit to perform a brightness simulation. According to the result of brightness simulation, an image is compensated and eventually displayed after being compensated so that a human being can perceive a consistent brightness.
The above-mentioned method can ensure the display quality of most images as well as reducing power consumption. The method, however, may have a problem when there is a gray level abruptly-varying portion in the image to be displayed. Usually, the gray level abruptly-varying portion can not be normally displayed. Examples of the image including a gray level abruptly-varying portion include a night image as shown in FIG. 1A and an annular eclipse image as shown in FIG. 2A. For this kind of image, since the gray level of this kind of image is quite low as a whole, the adjusted backlight brightness is quite low, and thus the image needs to be compensated considerably, so that the brightness perceived by a human being is not changed quite a lot. However, since the gray level of this kind of image is quite low as a whole, even if the pixel value of the image is compensated up to the highest gray level 255, the dramatic reduction of brightness caused by local backlight adjusting cannot be fully compensated. Meanwhile, a color error may occurred due to overcompensation and cause a clipping phenomenon as shown in FIG. 1b. On the other hand, if the gray level abruptly-varying portion in an input image crossing different backlight sub-regions, as shown in FIG. 2A, since the average gray levels of the pixels in different backlight sub-regions are different, the brightness of different backlight sub-region s are inconsistent, then the phenomenon of edge breakage as shown in FIG. 2B may occur, which will affect the eventual display performance.